Binding of molecules to a functional layer may be studied e.g. by attaching fluorescent or other (e.g. radioactive) labels to molecules. The labeled molecules may occupy active sites of the functional layer, and the amount of labeled molecules attached to the functional layer may be monitored based on intensity of fluorescence excited in the labeled molecules. However, it may be difficult or impossible to attach labels to certain types of molecules. The labels may also have an effect on the chemical and physical properties of the labeled molecules.